A field winding type synchronous motor is a synchronous machine and several methods of starting it up are known.
A typical example is a method using an inverter. The method using an inverter makes it easy to start a field winding type synchronous motor, because the motor revolving speed can be adjusted to a variable speed when the motor is started. However, in a case where there is no need for variable speed motor run after the motor is put into synchronization with an electric power system, the inverter would be used only when the motor is started and, thus, the initial cost for the inverter and its installation or the like would become an excessive burden.
As a method of starting up such motor without using an inverter, there is a direct online startup method (DOL startup). DOL startup is a startup method similar to full voltage startup of an induction motor. Despite the synchronous motor, DOL startup starts up such motor by taking advantage of the characteristics of an induction motor only when starting up it. In the field winding type synchronous motor, usually, rotor's field windings are excited by a DC current sourced from an AC brushless exciter. DOL startup makes the field windings disconnected from the AC brushless exciter and puts the field windings in a short-circuited state when the motor is started, in order to gain the characteristics of an induction motor. Besides, a discharge resistor DR is connected to a short-circuited circuit in order to reduce a startup current.
However, the discharge resistor DR is a device that is needed only when the motor is started and, inversely, comes to create a loss when the motor is running steadily at a synchronous speed, which results in a decrease in the efficiency of the motor. For this reason, the discharge resistor DR has to be disconnected when the motor is running steadily. Besides, to put the motor into synchronous run, it is needed to take steps for switching to a DC current sourced from the AC brushless exciter when the motor has accelerated nearly to asynchronous speed after it is DOL started.
In the circumstances as above, with regard to circuitry for switching from startup to synchronous run of the field winding type synchronous motor, various circuitry arrangements are under study; for example, those are known that are described in Japanese Published Unexamined Patent Application No. Sho 59-222087, Japanese Published Unexamined Patent Application No. Hei 06-343250, and Japanese Published Unexamined Patent Application No. Hei 03-078478.
Among them, in a circuitry arrangement as described in Japanese Published Unexamined Patent Application No. Sho 59-222087, a connection from an AC brushless exciter and a rectification circuit to field windings is made via a thyristor. A discharge resistor DR is disposed in parallel with the field windings. In this circuitry arrangement, the thyristor and a gate circuit serve as a circuit for switching the motor to synchronous run.
In a circuitry arrangement as described in Japanese Published Unexamined Patent Application No. Hei 06-343250, a circuit presented in FIG. 6 attached to this document is the circuit that serves to disconnect a discharge resistor DR and switch the motor to synchronous run. In this circuit, the discharge resistor DR is disconnected by a thyristor 24A. Motor startup with the field windings being short-circuited generates an induced current. The thyristor 24A and a diode 24B are provided to conduct this induced current to the discharge resistor DR. Upon switching of the motor to synchronous run, a DC current flows. But, because the thyristor 24A is turned off and the diode 24B is placed in an orientation to block the DC current, the discharge resistor DR is put in a disconnected state. In this circuitry arrangement, a circuit for switching the motor to synchronous run is a thyristor 2a. 
A circuitry arrangement as described in Japanese Published Unexamined Patent Application No. Hei 03-078478 is characterized by being provided with a device for positively detecting that the motor has switched to synchronous run.
In the case of the circuitry arrangement as described in Japanese Published Unexamined Patent Application No. Sho 59-222087, there is no circuit for disconnecting the discharge resistor DR. By turn-off of the thyristor, a DC current is conducted to the field windings and, at the same time, this current is also conducted to the discharge resistor DR. Consequently, as noted previously, even in the synchronous run state of the motor in which a DC current is conducted, the current continues to flow to the discharge resistor DR, thus creating a loss and resulting in a decrease in the efficiency of the motor.
In the case of the circuitry arrangement as described in Japanese Published Unexamined Patent Application No. Hei 06-343250, disconnecting the discharge resistor DR and switching the motor to synchronous run are performed by a gate control circuit 23 for the thyristors. Four thyristors exist in this circuitry arrangement. Accordingly, the gate control circuit 23 needs to have as many signal transmitting ports as the thyristors and the size of the gate control circuit 23 becomes larger. When this circuit is placed in a rotor, it would occupy a large mounting area. Besides, for the thyristor 24A for disconnecting the discharge resistor DR, there is a possibility that a signal is input to its gate because of noise or malfunction of the gate control circuit 23 to turn the thyristor on, thus conducting a current to the discharge resistor DR during synchronous run of the motor.
In the case of the circuitry arrangement as described in Japanese Published Unexamined Patent Application No. Hei 03-078478, because a discharge resistor DR does not exist in this circuitry arrangement, it is anticipated that, during the DOL startup of the motor, a current increases, causing a large disturbance to the system.
By the inventions disclosed in Japanese Published Unexamined Patent Application Nos. Sho 59-222087, Hei 06-343250, and Hei 03-078478, it is considered too difficult to reduce a startup current and positively disconnect the discharge resistor DR after the motor is put into synchronous run.
The present invention is intended to provide a field winding type synchronous motor having a discharge resistor DR included in a field circuit, provided with circuitry capable of switching the motor to synchronous run and disconnecting the discharge resistor DR, reducing the possibility that the discharge resistor DR is reconnected because of disturbance or the like after the motor is put into synchronous run, and allowing for downsizing of the circuitry.